But, now that I had to get that out of my system...
[SERIOUSLY=On]
Tennis Balls or any type of cushion, even a handgrip from say a Lawn mower handlebar can add a level of capacitance - well, I can't say it truly as a capacitor, but the effect is similar. The material is an insulator, which will generate a "bump" in the field as it propagates thru the antenna and also to free air. So they are correct to state that it will show a change in SWR due to this effect.
You can also do this - although expensive waste if you're on a budget - just think this thru first before you decide to prove right or wrong...start to remove the outer insulating cover to the antenna - like a Firestik and note the effects.
- Near the bottom and near the top of the whip has the greater affects of SWR changes than the tightly - more spreaded winds in or -in-between the load and base of the whip.
They have to change the length of wire in the antenna - the number of winds per-se - to offset this effect of insulated surface gap to free air.
But in light of 102" whip and Tennis Balls - Whether you're a Djokovic or Murray - a Federer or just plain Fed-Up with my jokes - Ahemn... I do understand ... But, to keep it simple - Tennis Balls or any sort of insulation covering a small region of the radiative surface of the antenna - regards to the more true the length of whip is to the fraction of the wavelength you wish to tune to - you suffer less Reflective and Reflection issues regarding SWR than you have with isolative objects attached to your loaded coil antennas that are physically shorter than their expected tuned wavelength.
So that means using a Tennis Ball on the 102" whip is less impact on SWR - than to use it without and wind up having several hundred to on up to thousands of dollars in DAMAGES caused by not using it to keep your vehicle from becoming...